The present invention relates to a method for drilling underground inverted arcuate paths and installing production casings, conduits, or flow pipes therein.
Techniques have recently been developed for installing production casings, conduit, or flow pipes beneath rivers and other surface obstacles without dredging the riverbed, digging a trench, or otherwise altering the obstacle. See, e.g. U.S. Pat. No. 3,878,903. Instead a pilot hole is first drilled from a position at or near the surface on one side of the obstacle to a position at or near ground level on the other side. As the pilot hole is being drilled a washover pipe of inside diameter larger than the outside diameter of the drill string is advanced behind the leading end of the drill string and surrounding it. See U.S. Pat. No. 4,003,440. Remaining aparatus is then pushed or drawn through the pilot hole to enlarge the hole to a larger diameter. The production casing is thrust into the hole immediately behind the reaming apparatus and follows it along the drilling path. See U.S. Pat. Nos. 3,894,402; 4,043,136 and 4,091,631.
Prior methods of drilling the pilot hole have utilized sections of drill string which are of uniform external diameter. When joined together such sections produce a drill string of uniform external diameter having no external protrusions at the joints between the sections or elsewhere along the string. Such joints are relatively weak, and the entire drill string tends to frequently stick during the drilling of the pilot hole.
Existing methods of advancing the washover pipe around the drill string, such as that disclosed in U.S. Pat. No. 4,003,440, provide the leading end of the washover pipe with a cutting edge which enlarges the pilot hole to a diameter equal to that of the washover pipe. Since no provision is made for supplying drilling mud to the leading end of the advancing washover pipe to entrain the cuttings dislodged by the cutting edge, the cuttings accumulate at the leading end of the washover pipe and inhibit its advance.
Prior methods of reaming the pilot hole and installing the production casing have used a single reamer and required that powered means be provided to thrust the production casing into the hole. Prior attempts to draw the ream apparatus or production casing through the hole with, for example, the drill string used in drilling the pilot hole, have resulted in the drill string knifing through the soil and the ream apparatus or production casing not following the original drilling path. In these methods it is also necessary to frequently interrupt the installation process in order to join additional sections of the production casing to the trailing end of the casing.